This invention relates to implantable prostheses and to methods for making them less susceptible to degradation when inplanted in vivo for extended periods of time. In particular, it concerns elastomeric polyurethane insulators for implantable electrical leads such as those used in cardiac pacing.
Background on biostability of implantable polyurethane elastomers and devices such as pacing leads can be found in Coury et al., "Biostability Considerations for Implantable Polyurethanes" Life Support Systems, (1987) 5, 25-39 and in Stokes, "The Biostability of Polyurethane Leads" Modern Cardiac Pacing, Barold S. Serge, Ed., Mount Kisco, N.Y.: Futura Pub. Co, 1985, pp. 173-98. In general, it is acknowledged that there are a number of mechanisms for degradation of elastomeric polyurethane pacing leads in vivo. One is environmental stress cracking (ESC), the generation of crazes or cracks in the polyurethane elastomer produced by the combined interaction of a medium capable of acting on the elastomer and a stress level above a specific threshold. Another is metal ion induced oxidation (MIO) in which polyether urethane elastomers exhibit accelerated degradation from metal ions such as cobalt ions, chromium ions, molybdenium ions and the like which are used alone or in alloys in pacing lead conductors.
It is believed that the ether linkages in the polyether urethane elastomers are susceptible to in vivo attack by these mechanisms. Unfortunately, the most desirable polyether urethane elastomers for pacing lead insulators are the most flexible polyurethanes which contain the most ether groups which are subject to ESC and MIO attack. For example, PELLETHANE 2363-80A is regarded as having nearly ideal flexural properties for pacing lead designs while PELLETHANE 2363-55D is regarded as being too stiff for many pacing lead designs. It is well known, however, that the 55D material (and other harder polyether urethane elastomers) has fewer ether linkages than the 80A material and is therefore superior in resistance to the identified mechanisms of in vivo degradation. Efforts have also been made to develop polyurethane elastomers for pacing lead insulators which have essentially no ether linkages such as those disclosed in U.S. Pat. No. 4,875,308 to Coury et al.; International Patent Application WO 92/04390; U.S. Pat. No. 5,133,742 to Pinchuk; and U.S. Pat. No. 5,109,077 to Wick. However, it is not yet clear whether any of these efforts to make a substantially ether-free polyurethane elastomer will provide a biostable polyurethane elastomer with mechanical properties as desirable as the mechanical properties of the PELLETHANE 80A now favored for use in polyurethane lead insulators.
U.S. Pat. No. 4,851,009 issued to Pinchuk employs a silicone rubber, typically a siloxane as a barrier coating over polyurethane to prevent in vivo cracking of the polyurethane. Unfortunately, the application of the silicone may require extensive treatments including the use of coupling agents, primer coats, exposure to a free radical initiator and the like. In addition, placing silicone over the polyurethane deprives the pacing lead some of the main advantages of polyurethane; the low coeficient of friction of polyurethane when wet that makes polyurethane leads easier to insert and maneuver when two or more leads are inserted in one vein and the toughness of polyurethane in resisting surface mechanical damage.
Additional background on the problem with polyurethanes can be found in Zhao et al., "Foreign-body giant cells and polyurethane biostability: In vivo correlation of cell adhesion and surface cracking", J. Biomedical Materials Research, Vol. 25, 177-183 (1991); and Zhao et al., "Cellular interactions with biomaterials: in vivo cracking of pre-stressed PELLETHANE 2363-80A", J. Biomedical Materials Research, Vol. 24, 621-627 (1990). Dolezel et al in "In vivo degradation of polymers" Biomaterials 1989, Vol. 10, 96-100, describes problems with polyethylene and silicone rubber in vivo.
It is therefore an object of the present invention to provide a polyurethane pacing lead insulator with improved resistance to in vivo degradation.
It is also an object of the present invention to provide a pacing lead insulator having excellent flexibility and mechanical properties.